Ultrasound imaging has been utilized for years to view features and structures obfuscated by overlying material, surfaces, etc. For example, ultrasound systems are commonly used to provide imaging of subsurface tissue and features (e.g., blood flow) in the human body.
Tissue Harmonic Imaging, “THI,” is a technique used in sonography to provide high quality images. THI relies on a phenomena whereby biological tissues (and/or other media) react to illumination by an acoustic wave (e.g., an ultrasound wave generated by an ultrasound imaging system) by generating second-order harmonics of the acoustic wave. Such tissue-generated second-order harmonics stem from the compression and decompression of tissue impacted by energy from acoustic sound waves transmitted into, or illuminating, the tissue. The resulting second-order harmonic signals propagate through the tissue and may thus be received by the acoustic transceiver of an ultrasound system. THI techniques utilize the received second-order harmonic signals to generate high quality images, such as to provide sharp edges, reduced clutter, etc. within the generated image. Harmonics higher than second-order are usually unimportant according to THI techniques because higher frequency harmonics rapidly attenuate as they propagate. Although higher order even harmonics could be used if desired, odd harmonics are not typically generated by the subject tissue and thus are typically ignored by THI techniques.
Because THI techniques rely upon the second-order harmonics generated by the tissue to generate the image, it is important that the acoustic wave illuminating the tissue be relatively free of second-order harmonics. That is, the presence of second-order harmonics not generated by the subject tissue may be emitted or created by the ultrasound system acoustic transceiver and electronics and degrade the quality of the image, such as by causing image clutter, decreasing edge sharpness, etc.
For example, according to the THI technique referred to as the “filter method,” the ultrasound system's receivers are frequency optimized (e.g., through combinations of both digital signal processing and receiver resonance) to the second harmonic of the acoustic wave. The resulting received second harmonic signals are processed into an image. In this technique, it is important to limit the amount of second-order harmonics generated within the transmitter as these harmonics interfere with the tissue-generated harmonics.
According to the THI technique referred to as “pulse inversion,” two successive ultrasound bursts are transmitted with the second burst being an inversion of the first burst. That is, mathematically speaking, the second burst is precisely equal to the first burst except that it is multiplied by −1. To receive the reflected signals, the ultrasound receivers remain broad-band and tuned to both the fundamental frequency and the second-order harmonic. Signals received from the first ultrasound burst are summed with signals received from the second burst. To perform this summing, both received signals are first aligned in time and are then summed (e.g., using digital techniques). The concept here is that if the two successive transmitter bursts are exactly equal but inverted, the summing of the resulting received signals will cancel except for tissue generated second-harmonics which are then processed into an image. The even-order harmonics generated by the tissue are not cancelled by the foregoing summing because the harmonic signals are generated in-phase, rather than with an inversion. In order for the pulse inversion THI technique to be effective, the paired burst signals must be equal and opposite, which has proven to be very difficult without implementing very complex circuits and/or highly linear transmitters.
Linear transmitter electronics utilized to minimize the second-order harmonics generated by ultrasound system acoustic transceivers apply a high amplitude sine wave to the acoustic elements of the acoustic transceiver and the acoustic elements respond by generating an acoustic signal (the acoustic signals from a plurality of acoustic elements combining to form an acoustic wave) having a low harmonic level. A problem with using this approach, however, is that it results in high power dissipation, mostly by heat in the electronics rather than by the power being transferred to the acoustic element. That is, transmitter circuits utilized to provide the high amplitude sine wave are somewhat inefficient resulting in appreciable amounts of energy being converted into heat rather than being applied to the acoustic elements for acoustic signal generation. This then has several negative effects, including thermal management problems and, in the case of a battery powered instrument, rapid battery depletion.
Techniques to limit second-order harmonic generation, without incurring linear transmitter power dissipation penalties, have also been tried. For example, techniques such as pulse-width modulation and stepped application of transmitters operating at different supply voltages have been employed. These techniques at the present state of the art, however, have not been found to be particularly advantageous because of their circuit complexity and the lack of adequate, extremely fast power switching devices.